Protein Isolation (Electrophoresis, Isoelectric Focusing, Chromatography) & Protein Analysis 🧐 🧪

Proteins are isolated to enable their study. The challenge lies in isolating a specific protein from thousands within a cell. Techniques exploit differences in protein features, such as size, charge, and shape. The initial steps involve cell lysis, followed by homogenization and centrifugation to create a protein-rich solution.

Electrophoresis uses an electric current to separate proteins based on their charge and size. Proteins, predominantly negatively charged, migrate towards a positive electrode (anode). Migration speed depends on the electric field strength, net charge of the molecule, and frictional coefficient. Polyacrylamide gel electrophoresis (PAGE) is a common method.

Native PAGE considers both protein size and charge, which can lead to different proteins migrating at similar speeds. SDS-PAGE (Sodium Dodecyl Sulfate Polyacrylamide Gel Electrophoresis) utilizes SDS, a detergent, to neutralize the protein's native charge. This denatures the protein and allows separation primarily based on size, simplifying analysis.

Isoelectric focusing is another electrophoresis technique that separates proteins based on their isoelectric point (pI). Proteins migrate through a pH gradient until they reach a point where their net charge is zero (their pI), at which they stop moving. This method exploits the incremental differences in protein charge.

Chromatography separates proteins based on their differential affinity for a stationary phase versus a mobile phase. Proteins with high affinity for the stationary phase move slower (high retention time), while those more attracted to the mobile phase move faster (low retention time). This principle is applied in various chromatography types.

Column chromatography separates based on polarity, with less polar compounds moving faster. Ion exchange chromatography leverages protein charge, attracting negatively charged proteins to positive columns and vice versa. Size exclusion chromatography separates based on size, with smaller proteins entering pores and moving slower, while larger proteins bypass pores and move faster. Affinity chromatography isolates proteins based on specific binding interactions with antibodies or receptors attached to the stationary phase.

After isolation, proteins are analyzed for their structure and activity. Structure analysis involves determining amino acid composition and sequence. Techniques include X-ray crystallography and NMR spectroscopy to visualize protein structure, and protein hydrolysis followed by chromatographic analysis or Edman degradation to determine amino acid sequence. However, hydrolysis destroys protein order.

Protein activity is typically assessed by measuring substrate concentration using spectroscopy. Ultraviolet spectroscopy is useful due to aromatic amino acid side chains in proteins. The Bradford protein assay, using Coomassie brilliant blue dye, is another common method. The dye binds to protein, changing color (from brown to blue) and allowing quantification of protein concentration; darker blue indicates higher protein concentration.